1. Field of the Invention
The present invention relates to a wiring structure for a semiconductor device, and more particularly to a semiconductor device having a multilayered wiring structure with a small parasitic capacitance.
2. Description of the Related Art
With the view toward establishing a larger scale of integration for semiconductor devices, there are strong demands for development of techniques for making the wiring width and wiring spacing more minute and for multilayering of wirings. Under these circumstances, reduction in the parasitic capacitance of wirings becomes important in the wiring technique together with reduction in the wiring resistance. The reason for this is that the parasitic capacitance increases with the increase in the wiring density or with multilayering of the wirings so that it becomes difficult to diminish the power consumption during dynamic operation of semiconductor elements. Furthermore, the reduction in the parasitic capacitance of wirings brings about an improvement in the speed of signal transmission between semiconductor elements which is effective in increasing the operational speed of semiconductor devices.
In order to reduce the parasitic capacitance, it becomes necessary to reduce the relative dielectric constant of the interlayer insulating film between the multilayered wirings. As such interlayer insulating films, there have been known inorganic insulating films like silicon oxide film as disclosed in "Proceedings of Third International IEEE VLSI Multilevel Interconnection Conference," 1986, pp. 100-106 or in "Proceedings of First International IEEE VLSI Multilevel Interconnection Conference," 1984, pp. 37-44, organic insulating films like polyimide as disclosed in IEEE Journal of Solid-State Circuits, Vol. SC-13, No. 4, 1978, pp. 462-467, and the like. However, since the relative dielectric constant .epsilon. of the former examples is on the order of 3.9 and that of the latter is about 3.0, these insulating films cannot be used as interlayer insulating films which satisfies the condition .epsilon. .ltoreq. 2.0 required for improvement of operating speed.
Besides the above, there is known a wiring technique called aerial wiring. In this technique, wiring is formed in the air without insertion of such an interlayer insulating film between metallic wirings. Although this aerial wiring technique makes it possible to attain approximately the condition of .epsilon. = 1.0, the technique has a drawback in that the wiring metal tends to generate sagging and deformation because of the insufficient supporting portions of the wiring, which causes short-circuiting between the wirings and breaking of the wirings, giving rise the wirings with poor reliability.